Objective-C - Private vs Protected vs Public

Question

I am hoping for some clarification on how Private vs Protected vs Public works with respect to class members when programming in Objective-C - I thought I knew the difference (I've added some comments to my parent class Person with respect to the same), but the fact that the compiler did not complain when I tried to access a private ivar/member of a parent class via the subclass now has me confused.

Here is my Parent Class:

/*  Person.h */  #import <Foundation/Foundation.h>  @interface Person : NSObject  {     //We can also define class members/iVars that are of type private     //This means they can only be accessed by the member functions     //of the class defining them and not subclasses     @private     int yob;          //We can also define class members/iVars that are of type public     //Public members can be accessed directly     @public     bool alive;      //By default class members/iVars are of type protected     //This means they can only be accessed by a class's own     //member functions and subclasses of the class and typically     //also by friend functions of the class and the subclass     //We can explicitly define members to be protected using the     //@protected keyword      @protected     int age;     float height;     } @property int age; @property float height; @property int yob; @property bool alive;  @end 

Here is my derived class Man:

    /*      Man - Subclass of Person     */      #import <Foundation/Foundation.h>     #import "Person.h"      @interface Man : Person     {         //iVar for Man         float mWeight;     }     @property float mWeight;      @end 

And finally, here is main:

#import <Foundation/Foundation.h> #import "Person.h" #import "Man.h"      int main (int argc, const char * argv[])          {             NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];              //Create a Person object             Person * aPerson = [[Person alloc]init];              //Create a Man object             Man * aMan = [[Man alloc]init];              //Let's attempt to modify our Person class members             aPerson.height = 5.11; //Protected             aPerson.age = 21; //Protected             aPerson.yob = 2010; //Private             aPerson.alive = YES; //Public              //Let's now attempt to modify the same members via our             //derived class Man - in theory, the private members should             //not be accessible by the derived class man             aMan.height = 6; //Protected             aMan.age = 26; //Protected             aMan.yob = 2011; //Private             aMan.alive = YES; //Public             aMan.mWeight = 190; //Protected member of Man Class              [pool drain];             return 0;         } 

Shouldn't the compiler complain why I try to access aMan.yob above? Or by using @property & @synthesize (i.e. the setter and getter methods) have I essentially made that member protected and therefore accessible to the subclass?

Answer 1

The usual trick is to create a class extension inside the .m file and put your private/protected property there instead of in the header.

//Person.m  @interface Person()  @property float height  @end 

this hides the 'height' property

Another trick is if you want to create a readonly property is to declare it in the header as

@property(readonly) int myproperty 

but in the class extension as readwrite which allows your .m to modify the value using the getter/setter

@property(readwrite) int myproperty